R. F. Kazarinov et al. (Soviet Physics-Semiconductors, Vol. 5(4), p. 707 (1971)) predicted the possibility of amplification of electromagnetic waves in a semiconductor superlattice structure. Since the publication of this seminal paper, the feasibility of a unipolar quantum well semiconductor laser has been considered by many workers in the field. See, for instance, S. J. Borenstain et al., Applied Physics Letters, Vol. 55(7), p. 654 (1989); Q. Hu et al., Applied Physics Letters, Vol. 59(23), p.2923 (1991); A. Kastalsky et al., Applied Physics Letters, Vol. 59(21), p. 2636 (1991); and W. M. Yee et al., Applied Physics Letters, Vol. 63(8), p. 1089 (1993). However, to the best of our knowledge, the prior art does not disclose observation of lasing in any of the proposed unipolar structures.
Those skilled in the art are aware of the advantages potentially offered by some types of unipolar injection laser. Among these are a frequency response that is not limited by electron/hole recombination, a narrow emission line because the line-width enhancement factor is (theoretically) zero, and a weaker temperature dependence of the lasing threshold than in conventional (i.e., bipolar) semiconductor lasers. Furthermore, appropriately designed unipolar semiconductor lasers can have an emission wavelength in the spectral region from the mid-infrared (mid-IR) to the submillimeter region, exemplarily in the approximate range 3-100 .mu.m, that is entirely determined by quantum confinement. The emission wavelength can be tailored using the same heterostructure material over the above mentioned wide spectral region, a portion of the spectrum not easily accessible with diode semiconductor lasers. Furthermore, unipolar lasers can use relatively wide bandgap, technologically mature materials (e.g., utilize GaAs- or InP-based heterostructures), without reliance on temperature sensitive and difficult to process small bandgap semiconductors such as PbSnTe. Such unipolar lasers could, for instance, be advantageously used for pollution monitoring, industrial process control and automotive applications.
Prior art proposals for unipolar quantum well semiconductor lasers typically involve use of resonant tunneling structures. For instance, W. M. Yee et al., (op. cit.) analyzed two coupled quantum well structures, each of which contains an emission quantum well sandwiched between energy filter wells, the coupled quantum well assembly sandwiched between n-doped injector/collector regions. The energy filter wells, respectively, have only one quasibound state (E.sub.1 and E.sub.3, respectively), and the emission quantum well has more than one quasibound state, with intersubband transitions taking place between two of the states (E.sub.2.sup.(2) and E.sub.2.sup.(1)). When, by means of a applied electric field, E.sub.1 becomes substantially aligned with E.sub.2.sup.(2), electrons from the injector can resonantly populate E.sub.2.sup.(2). If, at the same applied field, E.sub.3 is substantially aligned with E.sub.2.sup.(1) then the latter can be resonantly depleted into the collector. If the time constant for the former process is longer than the time constant for the latter then a population inversion in the emission quantum well could, at least in principle, be achieved.
As those skilled in the art will appreciate, the optical output power obtainable from a structure of the type analyzed by Yee et al., (i.e., a structure that comprises a single set of coupled quantum wells) is typically too small to be of practical interest. In principle this shortcoming can be remedied by provision of a structure that comprises a multiplicity of said sets. However, it is known that, for fundamental reasons, lasing typically cannot be achieved in such a structure. For instance, application of a voltage across such a multi-quantum well structure that comprises doped regions will typically result in a non-uniform field in the device, with attendant negative resistance and instability. See, for instance, K. K. Choi et al., Physical Review B., Vol. 35 (8), p. 4172 (1987).
In view of the considerable potential commercial and scientific value of a unipolar semiconductor laser, especially one that can be designed to emit in the mid-IR spectral region, such a laser would be of substantial interest. This application discloses such a laser, to be referred to as the quantum cascade (QC) laser.